The Fabricator - November 2013

manufacturers still have reservations about applying a water-based product directly to metal. High-solids are liquid coatings that have a solids content of at least 65 percent, which means minimal solvents are present. But fewer solvents also means that the coating is more viscous. ;at has led to the development of multipart application systems ( referred to as 2K systems if two parts are mixed, 3K if three parts are mixed, etc.) that are formulated to be mixed only seconds before application.

All finishers that apply liquid coatings spray to waste. ;e overspray can’t be reclaimed. If filters are used to capture the overspray, the finisher has to dispose of the filters according to regulations established by local authorities.

Powder. ;e powder booth does not require an exhaust. As stated previously, if any VOCs are emitted during the powder coating process, they are typically very low.

Powder coatings can be recycled with the right reclamation equipment. Companies looking for Class A finishes have reclamation systems that depend on thorough cleaning and maintenance because any cross-contamination of reclaimed material ruins the original material’s ability to deliver a specific color.

Again, disposal of unreclaimed powder coating material is governed by local regulations. In some instances, local law may require disposal in sealed containers or require that the powder coating material be baked into the form of a brick for disposal in a local landfill.

What Are the Characteristics of the Final Coating?

Liquid. When someone talks about a Class A finish, typically used to define the coating quality on a new automobile, most people think of liquid coatings. Specialty finishes, such as those incorporating metallic flakes, also are possible in liquid coatings.

Durability has improved over the years as well, especially with the emergence of 2K coatings. ;ese are typically used on large metal parts that can’t be powder-coated because they can’t fit into a typical booth and oven setup.

Liquid coating finishes can be applied in various thicknesses. Obviously, the more mils applied, the better the protection. In many instances, a manufacturer, such as an automaker, will seek to balance maximum protection with the minimum amount of paint mil thickness.

Powder. Powder coatings o;er the same characteristics that come with liquid coatings.

Properly cured powder coat-
ings can o;er superior protec-
tion against chipping, scratching, UV rays, and cor-
rosive elements. ;is is why powder coatings are
often specified to coat metal products destined for
outdoor use.

Material development has progressed to the point where powder coatings can deliver a Class A finish. In fact, a major European automaker and a U.S. motorcycle manufacturer are using powder coatings for their clear coating. However, many manufacturers, including automakers, still prefer liquid coatings for that topnotch finish.

Standard powder coating finishes are applied in the thickness range of 2 to 4 mils. Specialty finishes like a hammertone or a texture are usually 3 to 5 mils thick. Functional coatings can be 10 to 40 mils thick.

What Type of Cleaning Is Necessary Before Parts Are Coated?

Liquid. A lot of fabricators simply wipe the part clean with a rag soaked in solvent. Others rely on a wash of some kind with pretreatment chemicals.

Solvent helps to prepare the metal surface because it has aggressive cleaning action and actually prepares the surface for adhering to the paint. New coatings that have less solvent content may require much more formalized pretreatment processes to achieve a quality finish.

Powder. Pretreatment is critical when it comes to powder coating. If a powder coating is going to last, the part needs to be thoroughly cleaned.

Pretreatment can range from a simple abrasive
media blasting chamber to a multistage pretreat-
ment system with several chemical application
and rinse stations and an oven. (Parts have to be
dried and cooled before any application of powder
takes place.) Some of the pretreatment chemicals,
such as zinc phosphate, have to be treated before
disposal, but newer, environmentally friendly pre-
treatment chemicals have emerged in recent years
to ease the disposal hassles. ;e Environmental
Protection Agency, however, likely will seek chang-
es in the near future that will call for treatment of
all wastewater prior to discharge.

What Are the Basic Booth Requirements?

Liquid. Booths or stations used in liquid coating are typically made of metal. Whether in a manual or an automated setup, the paint is often sprayed to waste.

Fumes are exhausted outside during the painting process to keep the work environment in and around the paint booth free from the strong odors.

Powder. In some instances, particularly if batch finishing is occurring, a shop might use the same booth for both liquid and powder coating. But fabricators need to keep in mind that if they choose to powder coat in a liquid application booth and exhaust overspray outside the building, employees could be walking out to cars with all new finishes at the end of the shift on a very hot day— depending on where that powder overspray landed. For the most part, however, filters and correct powder coating technique should prevent most powder overspray from going outside.

If a company is doing any kind of high-volume powder coating, it should consider a reclamation system. Single-color reclamation systems are car-tridge-based and typically made of stainless steel. Multicolor systems, which are designed for fast color changeout, are plastic, making them easier to clean. Sophisticated fan setups keep the overspray in the booth, and the polymer-based interior walls prevent the powder from adhering to them. ;e overspray is collected and recycled for another application.